[deadlyhifi]

Have a look here on how to exclude a forum from the front page loop, whilst keeping the post limit intact.

limit forums included in latest discussions

Note the core hack required to keep the paging numbers correct.

Maybe you then need to write another small custom query to bring out just forum 17, as I’m not sure you if you can have two loops. (perhaps the WP multiple loop info would give some insight into this: https://codex.wordpress.org/The_Loop#Multiple_Loops)