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It has been a long time since I wrote on this forum. My LFS will be getting some Bagrichthys macracanthus and thinking of getting some. I have a 140 gallon tank with loricariidae in it. I also have a full grown Pseudomystus siamensis in it and I'm worried that it might not like the now tankmates. So my question is will the Pseudomystus siamensis harras the Bagrichthys macracanthus because they are quite similar looking fish.

P. siamensis is a rather bold fish. B. macracanthus is a timid fish.
Given the fact that your siamensis is fully grown and has been on its own in that tank, I think it'll give the macracanthus a (very) hard time. So the thing you were afraid of will happen.
Best thing to do imo is get the siamensis out and add a small group of macracanthus.

I was afraid of that. It's a pity to get rid of the bumblebee becouse it's my newest adition and they are very rare here in Finland (not as rare as the Lancers thought). I have a another 50 gallon tank, but i doubt it's to small for him/her. The bumblebee seems so nice and cant imagine it killing other (big) fish. It sheres a hollow log/driftwood with a L-91. Then agein when I added it to the tank my Brachyrhamdia marthae died within weeks. Maybe he/she was harrasing it. Are the Bagrichthys macracanthus more active during the day then the bumblebee? My bumblebee rarely comes out even for food. If the foods drops really close to its hideout it quikly gets it but it dosen't move around the tank when the lights are on. Will the Bagrichthys macracanthus be different?

Nope, the lancer will behave pretty much as you describe for the bumblebee. I have kept Pseudomystus (but probably not P. siamensis as I remember) successfully with lancers in the past. My impression was that the Bagrichthys was bolder than the Pseudomystus.

I have on the whole found lancers to be a little delicate - certainly when 1st imported. Siamensis i have found to vary considerably from individual to individual whereas stenomus is a bit quieter.