National Electrical Code Articles and Information

1. Yes. You can use them with conduit or tubing. In fact, a reference to this practice is made in 314.16(C).
    

2. It's an elbow fitting intended to enable installing of the raceway; the contained conductors cannot contain splices, taps, or devices [314.16(C)].
    

3. Twice the cross-sectional area of the largest conduit or tubing to which it can attach [314.16(C)].
    

4. That would be the maximum number permitted for the conduit or tubing to which it is attached, per Table 1 of Chapter 9 [314.16(C)].
    

5. In a rigid and secure manner [314.16(C)]. Typically, a conduit body has screw holes or screw flanges for this purpose. Screw or bolt the conduit body in place. The conduit or tubing cannot serve as the support; you run conduit or tubing from securely mounted box or body to securely mounted box or body.