If you do decide to do YCHs/adoptables, I'd be very interested. Your artstyle is very lovely! I've done a few in the past for here and the Arena (and
plenty more back in my old deviantArt days lol), so I can give some advice. Firstly, only take a few at a time as not to overwhelm yourself (I usually do just 5 at a time). It's also helpful to have a form for people requesting to fill out. Make sure to state what you are/ aren't willing to draw (i.e. if you're only drawing official ponies, if there's certain subject matter you're uncomfortable with drawing, that sorta thing). Finally, don't be afraid to limit how many requests a single person can make. Some will see "FREE ART" and get a bit too greedy.
YES!!! Whenever you upload any of your artwork to the Internet,
ALWAYS put a watermark on it. I've had my artwork stolen/reposted without my permission in the past, before I started adding watermarks to everything. It's a very icky feeling. Hell, every time I make forum adoptables, I put "please ask me before reposting offsite" in the OP, and yet I've
still caught folks reposting them without my permission.
So, yes, to save yourself a headache, always use a watermark.