So, first and foremost, Target Lock is an auto-include on Riptides. Riptides, like most of our stuff, likes to keep moving as much as possible. The Riptide needs a way to ignore the movement penalty for shooting its main guns (both of which are obviously Heavy). So, in my humble opinion, if you're taking a Riptide, you're taking Target Lock on it, no ifs, ands, or buts.
As for the second SS choice, to me it depends on your main weapon choice. If you are running a HBC, you're gonna want an ATS with it. Since we're only damage 1 on the HBC, we need all the help we can get with it, so taking that AP to a -2 is the best option. This also heavily benefits SMS giving it AP -1. While it doesn't really help PR all that much, I can say from experience that FBs are hilarious with AP -5. But, at the end of the day, your SS choice needs to reflect your main gun choice, not your secondary weapons, so don't worry too much about those.
As for the IA, I would say pick up Stim. The IA doesn't need the help from an ATS, and nothing else really helps boost its killing power. So, in this case, we're generally better off boosting the Riptide's survivability. A 6+ ignore damage doesn't sound like much, but its better than nothing, especially when it allows us to have a chance of ignoring the mortal wound from the Nova Reactor.