Chelib

You can give an aircraft type some tags to do something like this although it behaves a bit strange. I think they require Landable=yes and the same tags that flak tracks use to carry infantry. If you load infantry units into this aircraft and the aircraft has a Primary weapon, telling the aircraft to attack something will instead make it paradrop all the infantry inside around the target. This can also work by using force fire (CTRL click). To paradrop vehicles, you need to give it Carryall=yes and make it pick up the vehicle. IIRC, paradropping tanks can cause bugs with the tank's size. Carryall logic is also pretty broken in default YR, you need Ares (https://ares.strategy-x.com/) to get carryall logic working

It's not possible to mix animations between units. The numbers in artmd.ini refer to the frames of the SHP file. If you want to give boris swimming animations, you need to extract boris's SHP and the seal's SHP, copy-paste the swimming frames from the seal's SHP into boris' using OS SHP editor.