The legs use a proprietary peg system at the connection between the waist and hip, but the connection at the hip and thighs is a standard PC-116 C-Type socket, which may be compatible with other kits that use the part or ones with similar dimensions in that location or have a peg that's the same size. Additionally, the waist connector is also a PC-116-C peg socket which I believe is the standard size used in the waists of a lot of early HGs (but don't quote me on that). You can look at the instructions on the product page to double check and cross reference with the manual of whatever kit you're looking to combine it with. You can also use the official Bandai manual repository at https://manual.bandai-hobby.net/ for just about every Gundam/30MM kit since 2000 if that helps at all (though it's mostly in Japanese so unless you're fluent you'll need to experiment with the settings to figure out what they do or just brute-force translate the entire page with your web browser).

They unfortunately can't fit onto any existing base without serious modification. If you want a model with funnels that can, try the RG Sazabi/Nu, RE/100 Jagd Doga, or Nu/Hi-Nu Ver.Ka; the former two are compatible with the yellow and blue versions of the FigureRise Jet Effect respectively, while the latter three all include some kind of clear parts that allow the funnels to be displayed in flight connected to a base or the model itself