1) Welding doors is more dangerous to the players than helpful in a situation where you need to leg it, as ZEDs seem to have a tendency to teleport away from behind the welded door and to the other side, and around the corner just ahead of you. Which means you just welded up your escape route and you are now surrounded. GG, eh?
Instead it can be more useful for the last one in the retreating group to just slam the door in the face of the chasing horde (just tap 'E' when running trough the door arch). This will buy the team a precious second to gain some extra distance, for reloading or healing. This will also keep the door usable for later, and as an added bonus stops Husks and Hans from shooting at you (they either have to wait for a zed to open it or walk up and open it themselves) for a little moment.
2) Support Frag grenades have a fairly large blast radius (level dependent?) and fire off shrapnel in all directions, making them extremely effective against any large group of soft, fleshy enemies as they will just tear everything in the range into shreds.
3) Berk (AKA Zerker) grenades are really only good for canceling out Scrake/FP Enrage, and also to prevent them from enraging. I think they need more than one grenade to achieve this effect though, probably 2 or 3 for an effective stasis effect, depending on difficulty and the target. For anything else the EMPs are not too great so they should be saved for real emergencies involving Scrakes and/or FleshPounds.
4) Commando HE grenades are most effective against Fleshpounds, and should not be wasted on Scrakes. This is because Fleshpounds take additional damage from Explosive damage and Scrakes take reduced damage.
5) Do not underestimate the T2 Bullpup as a secondary weapon. It has a large magazine, negligible recoil, both single fire and full auto modes, and can reliably and consistently score headshots even over long distances trough the holographic sight with very little recoil recovery time. Skilled commandos who rely on skillful aiming and headshot kills over raw damage output may prefer this over the AK-12.
6) As a Commando, and when dealing with a Scrake, make bloody damn sure your assault rifles are both fully reloaded and you have got some distance between yourself and it before the moment it decides to tear you a new one (this happens when his health is around 50% mark). Also, when you unload, do not reload but switch weapon instead to release a steady flow of lead into his face.
7) Any Veteran Commando worth their salt knows this but I will share this tip with the new recruits: Always, always, ALWAYS leave at least one round in the chamber before you reload. This will save you a *lot* of time reloading because you won't need to pull the hammer back. Trust me. That second you saved just saved your bacon over and over again.
HOWEVER... That last one (7) was not a perk-exclusive tip. It also works for many other perk weapons, not just Commando weapons.