Combine cards before using them to train other cards. I think uncommons are like 40xp for each single,but 85xp if you combine it. So even if you got no intention of using the card in a game,still try to hold on to some cards so you can combine it and make them a pro and then just get the extra XP when training up other cards.
I wouldn't get "proper pros"(meaning both singles were fully trained before you combined them) for rare or lower tier. Super Rare and higher though you should pro them. If you got no intention of training/using the card like I said,then just combine them before you use that card to train up some other card.
You get no rewards for completing team collections,so holding onto cards isn't really useful.
Even if you got no cards you currently want to train up, if you can,just pro the rare cards(without training it) and then just train up abunch of cards. So it'd be like you got abunch of cards waiting in your deck to train up super rare or higher tiers if you have duplicates of a card. Crawford for example,since you know for sure you'll be getting another one once you hit the next ladder reward, he'd be a card you should train up,and then maybe train up an uncommon or rare card,so by time you get the next crawford card,you just use that trained up uncommon or rare,on crawford,and he'd be trained up almost right after you get him.