I don't think I've ever played medic in a highlander, but most of the rules of playing medic in 6v6 should still apply just the same. I would say the only thing that is a bit different is that you constantly need to check your back for spies, and you need to be careful with carelessly walking out in the open since you can get sniped at practically any point. A really important aspect dealing with the last sentence is if you know exactly who plays what class on the other team (especially spy, sniper, and demo), you can start playing with less worry and more freedom until they spawn and get in position again.
For positioning, hiding behind your heavy should be pretty straightforward. He's a giant 450 hp meatshield, and you should be able to hide behind him or around a corner to minimize much of the damage directed at you. Heavies are pretty much unbeatable at close range unless there's tons of focus fire against the heavy, so as long as the heavy as good reaction time and can turn around to protect you if needed, you should be just fine. If your heavy goes down, just run to a soldier or anyone nearby that can do a decent job protecting you until your heavy spawns.
Calling the medic's uber isn't as hard as you think, you just need to remember to do it. If the medic builds the uber as fast as he possibly can, it takes 40 seconds to get 100% ubercharge (you can time this by seeing exactly when he spawns). Compare this to how much uber you have and communicate it. In example, if the medic spawns and you have 60% ubercharge, simply say "their medic just spawned, we have 60% on them." Likewise, if medic just spawned and you died, you can have a rough idea in your head of how much uber they have when you spawn, saying something like "they roughly have 40% on us." You could technically time their uber by the game clock, but if someone caps a point, the game clock resets. As long as you generally know where the uber stands, that's important. Also, don't forget that building kritzkrieg takes 30 seconds instead of ubercharge's 40 seconds.
For calling in general, just communicate where you are a bunch of the time. It's so much better to have a medic that won't shut up about his position and his ubercharge percentage than a medic who is pretty quiet. If your teammates communicate pretty well, then there's not much need to communicate any action going on unless it directly involves your heavy and you, because you don't wanna clog up ventrilo/mumble with too many people talking at once. Plus, it's not always easy to see what's going on around you when you're constantly watching out for spies and hiding from snipers. There's eight other people on your team, so I'm pretty sure most of everyone can do a good enough job communicating the things you don't see. Also, don't forget to say that you died or that you even dropped uber. It doesn't matter how embarrassing or unlucky the situation was, because it's better for your team to know that the uber you once had (which you now dropped) did not and will not happen anymore, or if you simply died without uber. Therefore, your team can readjust their position and gameplan accordingly.
tl;dr
-Check your back for spies quite often
-Don't blindly walk out in the open when a sniper is likely looking for a pick on you
-Hide behind your heavy when pushing because he does a fantastic job protecting you
-Hide behind corners and walls to minimize taking unneeded damage
-Compare your uber to the enemy uber from when you/they spawn
-Communicate your position a bunch so your teammates know where to go for heals
-Communicate when you died or when you drop uber
I'm not familiar with hl maps at all, but review the maps with your team so everyone is on the same page. Also, a big key to improve is not to get angry at yourself or in general. Once you become angry, you start to lose focus on what's going on and you won't improve at all.
I hope this helps. ;0