in PLO you have to be way more careful with the type of full you have. Taking your example of the 6's full, flopping the underfull with 66 on KK6 or the overfull on 655 is a huge difference. On KK6 you can easily be outdrawn by anyone holding a K, so you might not want to build up a pot too much because you'll basically be blind flying on most turns, especially oop.
So based on the number of opponents you might want to check your underfull and see what happens and even fold to heavy action behind you. Against one bettor behind you you might be able to check/raise it all-in, but don't expect to gii as a big favorite too often.
Let's say next hand you are holding A432 and the flop comes AA4. Solid overfull, but again you want to be careful playing it oop. If you pot it and get called, you will be guessing from the turn onward if your opponent spiked a better full and will never feel comfortable playing check/call to full pot bets.
So yeah, what you want to do is ask yourself if you want to build a big pot or not, and play accordingly. There is no shame in check/calling down the better hand and keeping the pot small. There is also no shame in folding a full house