Okay so full disclosure: I haven't so much as opened an art program in like 15 years (give or take). I downloaded Krita, opened it, and immediately felt overwhelmed and humbled. And even after HOURS of tutorials, I have SO. MANY QUESTIONS. Is Krita an okay program to use for making custom wearables (eventually, I'm waaays off from that ATM)? Is 300 ppi standard? When Subeta asks for a "transparent .png" all I have to do is make sure the background is at 0% opacity, right? When I save a file down as a .png, will it compress it? because right now even just an empty file (0 pixels) with the proper number of layers is 256k and I feel like I must be doing something wrong if the limit is 500k. Apologies for not knowing what the heck I'm doing, and thanks in advance for your patience (truly the best online community in my very humble opinion)

I use 120-200 ppi personally for artwork that I upload to web usually.

300 ppi is super detailed and for scanned in artwork( paper to scan) it's actually really good if you want to go in and re-trace the linework to clean it up, but for artwork that's uploaded to an upload image site such as photo bucket or imageshack it'll generally make the file too large for upload even when compressed down as a png. I usually scan stuff in at 120-200 ppi as a compromise since it's still detailed then, but doesn't make the file as large when uploading it to a web hosting site. For the record, as I've only drawn personal artwork and have never drawn custom wearables for subeta I can't give that much commentary on custom wearable standards as I'm not a cw artist though.

As for Kritka, I use photoshop so can't comment on it as I've never used it, but looking up infor on the program via the almighty google, if you uncheck the alpha filter I believe( at least according to what I found on the web) when saving it should be savable as a transparency. If you don't uncheck the alpha filter it's very likely it might still have a white background when uploaded as far as I can tell.