Can you wear baseball keepers' protective gear under your clothing in cricket?
AFAIK, yes it is allowed to wear baseball protective gear for cricket.
You are discussing about the protective gears under the clothes in baseball, ie, specifically about the leg-pads.
It looks more or less same as the cricket leg-pads.
Only the wicket-keeper and the fielders in short leg and silly point are allowed to use the leg-pads in cricket, but the leg-pads used by Wicket Keepers in cricket is slightly big and goofy than baseball leg-pads. Since the cricket ball is more hard than baseball. So, that type of pads are preferred by cricketers. No rules were block them to wear baseball leg-pads.
Yes, it is allowed,but after Umpire's consent (according to the law). But under the clothing, I don't think so. Helmets were allowed in a few cases (Dinesh Karthik, McGlashan).
Fielders at forward short leg or any other fielding postion closest to the pitch often wear shin guards(non-cricket gear/generic sports gear).
While, other fielders use a thick layer of bandages/tapes on fingers.