While primarily an acoustic guitarist I play mandolin, mandola, octave mandolin, 4-string banjo, etc. Jim Dunlop Tortex .73mm are what I've settled on after many years. They're a little stiffer than Fender .73mm, but not as stiff as the Fender .88mm which is the next step up. The Dunlops are bright yellow and can be seen easily if dropped. Would recommend not going any lighter as the next step down is a little 'floppy'.

'Celtic' and 'Bluegrass' style strumming are 2 completely different disciplines. One needs to be proficient in both. The 'bluegrass chop' is played on the 2 & 4 beats of a 4/4 measure and 2 & 3 beats of 'waltz time'. The use of full chords w/no open strings accomplishes this. 'Celtic' uses a more flowing rhythm approach w/more use of open string chords.

While the sound is more delicate in my experience it's the more difficult of the 2 disciplines because the subtlety of movement requires greater wrist control and the restraint of a lighter touch. Just MHO. Also, the mandolin is used mostly as a melody instrument in Celtic style. In bluegrss it's used as the 'snare drum' while taking 'breaks' and doing 'fills' in trade w/t fiddle, banjo and dobro.

Just pick out songs you like and learn to play/sing them on the mandolin. As your strumming improves you'll find yourself taking on more and more complicated songs. I play anything from 'Til There Was You', 'Pretty Woman', 'Ordinary Man', Tiptoe Through The Tulips', 'Old Joe Clark', 'Music For A Found Harmonium', 'Wayfarin' Stranger', 'Little Begger Boy'(Red Haired Boy in bluegrass cicles) and 'Lies'(a Stan Rodgers song). They all lend themselves to the mandolin very well. You'll enjoy steady improvement if playing/singing songs you like. Have fun...it's a lifelong process.