Stick a micro dart in your pipe and tell me how it fits. If the foam (not the tip, just the body) is a tight fit, then you have CPVC. If the foam has a lot of room on either side, and you can fit the head of the dart into the pipe as well, then you have PVC. Tell us which one it is and then you'll make enough sense for us to help you.
Regarding classifying mods in terms of difficulty, I agree with Omega. Though you make a good point that difficulty is subjective, the system could be based on general trends. I bet 99% of the users of this board will say that a nitefinder is easier to mod than a longshot, for example, and ideally, a rating system would not discourage people from attempting an ls mod, but merely get that trend across to them.
But there are only a handful, probably less that that, of guns that everyone could reach a single consensus on. The "trends" that you're talking about are either obvious in such a way as to negate the need to officially list them, or are an average based on a large amount of vastly different opinions. The latter would mean that my point was valid, and because there cannot really be a set difficulty for a mod, there is no way to accurately classify them based on how easy or hard one is. Badger and I both completed LS mods as our very first attempts at modding, and found it easy while others may find it too complex or confusing, for example. Some may screw up a Nitefinder mod while others do it automatically because to them it is obvious what needs to be done and how it is best executed. That fact is that difficulty is objective and therefore, apart from the commonly accepted "NF is easy" opinion, there really is no established ground for what the ratings would be. The best way to do it would be to make a poll for each gun, but that would be time consuming and still ineffective. The best way for someone to tell what they should do mod wise is to pick and complete a mod, judge for themself how difficult it was, and then decide from there what is within their abilities by looking for similar mods and procedures.
Mod potential is mostly restricted by the imagination of the modder. Of course, there are also the more versatile guns that will naturally be more useful for mods. Nitefinders are well-documented there, as are Longshots. SSPBs and SMDTGs are good for integrations because of their small size and respectable power.
It is assumed that someone who builds and attaches a silencer to their gun knows that it only works for stored-air weapons. Silencing a gun by stuffing it full of foam works to some extent on spring guns. But this is off topic.
You could always replace the plunger rods with a threaded rod or a steel/aluminum solid-core pipe if you're not sure. However, marine strength epoxy, JB Weld, Plumber's Goop, plastic cements (or plastic welds), and Fishin' Glue are all very likely to hold. If you want to reinforce the hold, try bonding some strips of aluminum flashing onto the sides of the plunger rods where they are joined (I.e. take a small strip of flashing and glue it down so that it is glued to one plunger rod, extends across the joint where the two rods meet, and then is bonded to the other rod on the opposite side as well). That gives you a lot more adhered surface and strength. The plunger tube itself is of the same plastic, but I can't help with the PETG ones.