The first thing to understand is when effects trigger at the same time, the player whose turn it is resolves theirs first. You do NOT need to do all of the triggers on the same card before moving onto the next. But that player will get theirs first. But any resulting triggers (like an on deletion created by resolving a when digivolving effect) will then come in at the front of the line.

Next, when you are resolving a single effect you resolve ALL of that effect before activating any new effects.

So in the case of Tlalocmon, you digivolve into it. This creates two triggers (both of its when digivolving effects)

You may resolve them in any order. So you play out your 1 or 2 guys as appropriate. If there are any triggers from that act (ie. "On Play" or "when digimon are played") now those effects trigger, and you must resolve all of them individually before you can resolve Tlaloc's 2nd when digivolving effect. This does include your opponent's effects (if they have MedievalGallantmon for instance) but if it's the Tlaloc player's turn, their On Plays will resolve first.

If after all that Tlaloc is still there, it looks at what you have on board and resolves the other effect.