Hey JP_36,
the longer your bit is, the more it will be bent and induce chatter. You will have the least problems if you always use the shortest bit possible. So if you want to mill a deep pocket, I would start with a short bit and use the long bit only when you reach more depth. Also the diameter of the bit is important. Unnecessary to say that a 6.35 mm bit is a little thin for the crazy length. The longer the bit, the larger the diameter should be to prevent bending and chatter. I would use nothing under 8 mm (Makita is sold with 6 and 8 mm collets outside of North America).
Also when milling wood, I would use a very high rotational speed (depending on number of flutes, I use a formula). And finally the depth of cut, the feed rate, and the stepover should correspond to what the hardness of the wood allows.
With a spindle with ER-20 collet, you could use up to 13 mm bit shanks (including half-inch bit shanks) and very long bits that will bend less.