1) I haven’t played college game yet but I’m curious how or if it interacts with pro game ? My plan is to start a career mode in Pro but at the same time I’d like to be able to monitor season progress in college. This way I could keep track of college players and their performance/stats before eventually drafting them. Can those 2 games be linked and how it would work ?
2) What about College players ratings once they become Pro ? Let’s say this RB had 90 for Speed in college game does he stay at 90 in Pro game or it gets reduced ?

You can link some of the data from the college game into the pro game... right now it's mostly standings and such, but we may look at expanding that now that both CF21 and PF21 are out. The college game generates a linked.json file that you point to from within the Pro game and anywhere linked data is available it will show up it the pro game. You'd basically open the college game, sim a week or progress a stage, then save and exit. When opening up the pro league you'll see the updated data.

For college players going into pros the CF game will generate a draft class each season and store it in the league file location so that when you're ready to import into PF21 you can. There is a checkbox on whether you want the game to autoadjust the draft class back down to normal rookie levels.. if you don't adjust then the college players come in with most of their ratings intact and unaffected. (There isn't a way for me to know whether people have manually created draft classes, or are using CF imports to the adjustment option is up to you)