Can Airush smart bar be used with other brands such as Slingshot? Because it s adjustable and no need for two bars with two kites 10m and 14m.

2013 Smartbar can as it's single front line flag out so is compatible with SS.
Line lengths are also adjustable, 24m, 21m or shorter!!!!

I just wanna make sure that whether or not I am gonna have any problem using smart bar with Slingshot RPM. Is it easy to remove the fifth line? Can I do it or it has to be done by a professional. Or can the store sellers do it? I am planning to use the bar with 14m and 10m RPM

There's a video on Airush.com about rmoving the 5th line,,,,takes about 3 mins.

Then it's just a 4 line bar so will be ok on the RPM. You might have to trim it a bit to get it set to how you like it,,,,then again it might be perfect straight away