Fuses in general

Okay, neither my flight manual nor maintenance manual say what kind of
fuses I should be using between the batteries and instruments. The
printing on one looks to be T500L250V, which I understand to mean 0.5
amps, slow-blow, 250 Volts. (It's 5x20mm.)

I know that to protect my wiring I need something in the 2 to 3 amp
range, but slow-blow or fast-blow? Does it matter? In choosing the
fuse, do I consider only the wiring to the instrument, or do I
consider the instrument itself? (If it draws 300 mA peak, I imagine
the fuse must be at least 600 mA.)

I need to replace the blown T500L250V and get some spares for the
others -- I just don't know what to order.

~ted/2NO (software guy)