Im lefty, and Ive been practicing my inside out slice serve... I get way more spin that a regular slice serve for some reason. to the point that ball practically makes a 90deg turn when it hits the ground. And the returner would have almost no chance I getting to it. Makes me wonder if it might be illegal if the opponent could never get to it
Tennis isn't famous (unlike maybe cricket) for banning things that give success - in fact the tennis serve actually (imho) has very few rules at all....
That said - two thoughts about the 'i/o' slice:
1. I've rarely, if ever, seen it used in competition or at the pro level. I think this may be due to questionable accuracy, and also...
2. It's a very readable serve imho, I can spot it quite early usually. One of the strengths of Pete's serve is often touted as the fact that he could hit very different serves with the same (similar) toss & motion.
That said: If it works for you, good luck to you....