Hi, welcome to the forum, methotrexate is quite an effective drug used in cases of rheumatoid arthritis. The detailed drug interaction with montelukast is not been reported or extensively studied. We take it as no harmful effects reported yet. Yes you can take combination but after consultation from your doctor.
There are few drugs you have to be careful or avoided when on methotrexate like antibiotics (penicillin, ciprofloxacin), proton pump inhibitors, pain killers like aspirin, other immunosuppressants. I hope this helped you. Take care and regards.
I did a bit of digging for you and I found this web site:
http://www.drugs.com/drug-interactions/methotrexate-with-singulair-1590-0-1654-1024.html
You can also ask your pharmacist to make sure they know everything about medication interactions.