You won't be able to connect to RC:RP if you're using SA:MP 0.3.7, you're required to have 0.3DL. And yea, you can use the SA:MP addon on SA:MP 0.3DL. However, much better option would be just removing the mods that are causing the problem, as SA:MP addon really just creates more issues than it fixes and alters some of the game's settings, such as not marking you as away when you ALT+TAB or making you unable to use /blindme (without fiddling with it more), and etcetera, which is not ideal.