Can you choose who you disguise as when disguising as a class with more than 2 players

In addition to learning the maps, there are a few other things to know:

• Sentry guns beep 1-3 times each time they change directions (so about every 3 seconds). Once for mini/level 1, twice for level 2, thrice for level 3. Generally, if you can hear a sentry's beep, you'll be in its range when you round a corner.
• As a Spy, you'll get a LOT of cheap deaths. Sadly, this is because of the Spy's shortcomings in order to combat the instant-kill backstab.
  • The Spy's Dead Ringer takes some getting used to, but in certain situations, using it (preferably while disguised as a teammate so they don't know you're a Spy) can get past a defensive line. Beware that the Dead Ringer makes a louder than usual decloak sound. The Dead Ringer is a "perfect" cloak for the first 6.5 seconds, so enemies who touch you won't make you appear as a team-color outline. However, they still can't move through you.
  • The Cloak and Dagger gives you some latitude by not decloaking when you don't want to, but it also has a very short cloak time.

Having said all this, you're still going to die a lot in Team Fortress 2. It's the nature of the game! The lower-HP classes in particular are specialized in some way, but lower HP is the drawback.

In order to avoid it I suggest to not try entering more than once in the door of specified team, if the first time fails selected random.

Anyway, if it happens that you got stuck, you can easily recover from this bad situation by opening console and write

retry

Retry command force a server reconnection and you access again the team selection screen. Sometimes, on busy servers with many people try connecting, this command causes to lose the slot.

For this reason, as suggested by @Dave McClelland in comments, is preferable use the command

spectate

because it fix the problem without forcing a re-connection.

If you really want to join RED or BLU team and reduce the probabilities to trigger that annoying behavior, I suggest to wait that half or more users on the server choose their own team, and then try enter your desired door.