Can couplers and nipples from different series be combined?
Do not rely on it. Only couple within the same series and size. Halves from different series sometimes engage mechanically but will not hold operating pressure safely and will damage the seal.
What do the characters in a number like 71-3C8-8RP mean?
The first block names the series, the second size and body style, the third the connection. RP, for example, denotes a parallel female thread. Size, connection size and thread form are also spelled out in the article technical data.
Can the coupling be connected under residual pressure?
Only with series designed for it. If the line is still pressurised, relieve it before coupling, otherwise the connection will not close or the seal is damaged. Whether an article may be coupled under pressure is stated in its technical data.