The way I see it, a guard post is there to prevent agents sneaking through your doors when a minion opens them, but they must still be able to get past somehow (so it's not OP). Your guardpost works fine for preventing access to a control room or your inner sanctum, or basically just being outside something, but I don't know how well it would work in, say, your standard corridor, especially if someone stacks up 8 in a row. Then, depending on the size of the enemy squad, they'd only get so far before being stuck and it would be too powerful. Additionally, there has to be some way (besides disguise) for agents to get through without just shooting your guards.
My vision of something like this would be a 1x2 building with a couple of small metal barriers on the 2 side facing away from the door and a chair behind it on the top. The guard sits on the top chair, and at higher levels the second guard stands at the ready on the bottom. Ex:
(the acutal distance between the Walls is 2, but I use four lines to illustrate better)
To prevent running out of space for people to fit through, and since most squads of agents and can fit in a 2x2 area, there should be at least a 2x2 free squares in front of a guardpost. Thus, in a 6x2 corridor you could have door, guardpost, empty empty, guardpost door, but not just 1 empty 2x1 space (which would could be problematic for pathing). Additionally, the metal barriers should only take up (at most) half the corridor space.
1. If agents come from the right, they just hack the door and walk through, or sneak through if a minion passes through while they're hacking, guards are supposed to prevent people going in, not out. (useful for agents 'attempting to escape' with the blue ring, you can just let them walk out)
1A. It is possible to have guard posts on both sides of a door.
2. If agents come from the left, and the security post is at level 1 or 2, they argue with the guard, taking the place of hacking. Minions can walk past, but the agents can't. Eventually, the agent will convince the guard to let him through (alternately, after a while high level agents will perform a judo chop and knock the minion out, but not kill him, then push a button and open the door. Agents will automatically do this if a disguised agent is detected while they are talking, which also prevents the disguised minion from being tagged.), but he will not walk through if a minion does. If there are no guards, the agents just hack the door and possibly walk through if a minion does (like what we have now).
3. If agents come from the left, and the security post is at level 3, they will argue with the first guard, taking place of hacking. Minions will walk up and be slowed down by the second guard, before allowed through, but agents won't walk through with them. Eventually, the agent will convince the guard to let him through (alternately, after a while high level agents will perform a judo chop on the first guard, followed by a roundhouse kick to the second, kicking them out but not killing them, then push a button and open the door. Agents will automatically do this if a disguised agent is detected while they are talking, which also prevents the disguised agent from being tagged.), but he will not walk through if a minion does. If there are no guards, the agent just hacks the door and possibly walks through if a minion does.
4. If agents come from the left, and the security post is at level 4, they will be automatically tagged for capture. If the guards are killed, the agents will attempt to hack the door like normal, and may run through if a minion passes through. Minions will be allowed through if they are going to work, but otherwise are not (this would be the setting you want outside your prison, power plant, or freezer, so minions don't just wander in).
5. If a disguised agent comes from the right, he may be caught if there are guards, otherwise he walks through.
Side note: Guard posts override the security level of a door from that side, so if you have a level 3 door and a level 1 security post, it will only have level 1 security on one side, but level 3 on the other, so your minions may get stuck inside. Also, the amount of time a guard takes to be convinced to open the door may be linked to his smarts, and arguing with him could be a psychological attack reducing it, which would be divided between both guards at level 3 or 4. The amount of time it takes before an agent just kung-fu's your minions should not be dynamic, they just do it after x seconds. If the agents have heat, they just shoot your guards and open the door instead of arguing.
A possibly researchable upgrade to this is a force-field door (standard 1x2). Which does the same thing except it's a force field. Instead of security checks at high level it scans with a sci-fi laser grid. This uses standard door security protocol (only yourself and ordered henchmen at level 3 and 4), and agents just hack the door regularly (or use some strange gadget to shut the force-field off). This may have a guardpost attached to it. If it does, the guardpost will open the door if convinced to open it, but if the guards are knocked out, the door will not be gadgeted open except possibly by a super agent. This combo at low level increases the chance that a disguised agent will be found (effectively a second chance since the guard glances and the door does a light scan. Additionally, agents with heat will not shoot the guards to open this door if there is no guardpost, they will just try to use their gadget to open it.