After my medial meniscus surgery it took quite a while before I was able to sit fully in seiza not alone do suwari waza. I would make sure it is fully healed and you are able to sit in full seize before doing much knee work. I was off the mat one month and it took a couple more before I was comfortable sitting all the way down. Medial meniscal tears are usually due to rapid flexion with external rotation of the foot. I did mine as a kid doing handsprings in tumbling. Aikido made it worse.
A good rehab program is essential. I used to wear a soft brace probably more for psychological comfort than as any preventative.