DIY duct work

Greetings, again! It looks like there is a boatload of information and knowledge on this forum so while I'm here, I'd like to run a few other questions by everyone. I am preparing to do the ductwork for the basement I am finishing. I followed the builders plans. Two smaller 11'x11' bedrooms, a 14'x24' living room, a bathroom, laundry area, and a small storage closet. I've gotten some advice from my neighbors who have recently finished their basement. This home was built 2 years ago and the duct work seems pretty straight forward. It's a split level in Minnesota with the basement 1/2 underground and insulated well. The main trunk runs the length of the basement splitting the basement in half while the return trunk runs about 1/2 the length of the basement. I'm going to run ceiling supply and low wall returns. 1 of each in the bedrooms and 2 of each in the living area. The builder had already ran a supply in the basement to one of the future bedrooms so I will be copying that with the other 3. 6" leading to a 10"x4" for the 4 supplies. I will be using oval duct for the returns. My questions are these: The ceiling is getting a little crowded already, I was planning on using flex duct for the two bedroom returns but it'll approximately be 15' of straight-ish run. Is this acceptable? Also, I've read varying opinions on tape and/or mastic. If I could get away with just using tape, I certainly would but I also want to do the job right so if both are recommended, that's what I'm going to do. Thank you so much!