is it possible to scale a single conference with known number of participants across multiple video bridges without using octo?
Nope, you need octo for that.
even when JVBs are co-located?
Is there any simple guide? I want to configure a couple of JVBs to support a max of 100 participants with max of 50 to be able to unmute their videos at most;
There is no such feature to disallow people unumuting video. You can set lastN to 50 which will allow a person to get up to 50 videos.