They WILL work with 30 and possibly 32T cassettte depending on your frame and chain length.here is a link from MTBR about this. Lots of guys use Road RD on their MTBs to save weight:As to short cage derailleurs. I'm using older ones fine with 30T cogs. Shimano likes to claim you can't do it (to sell you the longer cage models) but in reality the short cages can handle the bigger cogs just fine.
The only obvious reason to use a longer cage is the higher total chain capacity. Then again, us oldtimers are used to running 12-28 or 13-30 cassettes with 24-36-46 chainrings and Deore DX or XT short cage derailleurs and not minding the fact the chain drags/droops when in the granny ring and any cog smaller than oh a 21T. Because after that its just overlapping gears with what's possible in the middle ring.There's also the fact that in the early 90s 7speed days, shimano offered a 13-34 cogset at the HG50 (deore LX and below), 70 (deore DX) and 90 (Deore XT) levels even though they didn't market any derailleurs which could officially work with larger than a 32T cog. Simply more evidence that they'll lie thru their little japanese teeth to sell ya stuff if it suits them.
Shimano 105 R7000 Rear Derailleur Manual
The issue has less to do w/ how many teeth the derailleur will accomodate in the rear, but whether or not you'll have enough chain to accomodate the largest cog/large chainring gear combination. For example, I can run a 105 derailleur w/ a 12x32 cassette just fine, so long as i don't shift to the big ring/big cog combo, which will cause my drivetrain to essentially self distruct. At this point the chain becomes too short, causing the drivetrain to lock up.
You can compensate by adding extra links to the chain, but then you will have a whole lot of slop on the 39x12 gear combo (too much chain).Often this is not an issue w/ mountain bikes, since the largest chain ring is a 44t. But on a road bike, where the big chain ring is typically 52/53, you will have trouble using a cassette that large if you expect to be able to use all possible gear combos. As a point of reference, I have a cross bike set up w/ 38/46 chainrings and a 11x32 cassette using a shimano 105 short cage derailleur. Every gear works fine except for the 46x32. So, I just don't use this gear.