"can i do something like this?"
yes. you do not have to ask, and this was not my idea. here is a nice link to help you choose the license you want
"can i draw/model/write about/etc them?"
yeah as long as it follows the licensing rules
"can i make something f2u with your designs?"
please do. i allow sublicensing if you want to sell your work or whatever else, so pls check out the page for that
"can i sell the design as an adoptable/be inspired to make an adoptable?"
i dont like adoptables, and you cant sell the designs directly. but legally speaking, as long as your adoptable doesnt infringe on my design (like any other adoptable should), you'll be fine.
"am i able to barter for / buy one of these designs for my exclusive use?"
no. you cant remove a creative commons license. just steal it like the rest of us
"that design kinda looks like one of my ocs"
ur one of 8 billion, ur not a perfectly original and special guy