Okay, I'm going to throw my opinion out here for what it's worth, keep in mind that part of what I'm going to say is simply just a matter of my own personal taste and has nothing to do with me thinking that you're not on a good idea. Like the other guys who responded before me, I spend most of my time and a large part of my wardrobe is T-shirts...I've been accused of having nothing but, and too many of them. My excuse is that I'm clothing coordination impared and that I'm usually safe picking out what to wear if I can stick with T's and jeans...usually.
For starters, I think it's a great simple design...I would say you DEFINITELY should put Slipstream Serpents and your website on the front somewhere. I know you mentioned the other vendors/competition not wanting to advertise issue...here's where I see that. If another vendor likes your shirts and likes you well enough to purchase something from you, then I think they probably aren't going to mind wearing a shirt with your name on it and advertise a little for you...maybe not at shows, but it won't stop them from getting one from you to wear other places. Then again, we're onto that kind of personal opinion thing, and I myself have trouble seeing other vendors as competition so much as just fellow hobbyists.
Several of the vendors I'm used to being around do something like this and sell a few shirts here and there in the 10-12$ range, not sure what kind of cost you're looking at or your projected pricing. Most of the shirts I see are given away along with purchases of animals when the price tag for the animals was a couple hundred or more. The sizes I notice running out of most, are XL, then Large, then Small, then Medium. What it seems like I see sold most, are small sizes for kids.
As for the colors you have, I really like the top one you put up best, pink on black. Not sure why, but for some reason I'm just not fond of the white on anything other than the black shirt. If it was me, I'd try to narrow it down to three colors for shirts, black, pink (and I agree with pink is smaller sizes, probably small and medium for kids and ladies) and maybe the green shirt. I would pick just two colors to print with to keep cost down, black and pink, obviously with the black and pink shirts you would print with the opposite color, the third color shirt have done with both print colors...the reason I said green is I think that's the color shirt both print colors would work on best. I think blue might be a better third shirt color choice to go with both print colors, but you didn't show one of those. I really like the purple shirt, but I don't think pink would print on it very well.
Just kind of an OCD thing for me how shirts are usually done with logo's, although it may naturally draw other people to look at it quicker too...I would flip/reverse the image and then that would give you a spot top left to put your name where logo's are typically at.
Good luck with it whatever you decide to do!
Charlie 
