Inconel filler is a good choice. To avoid hot cracking (centerline cracks) , do limit the heat input to maximum 1.5 kJ/mm ; make sure the height of the root weld is greater than the width (maybe you will have to change your welding preparation, eg. a bigger opening angle) ; clean all surfaces : ni-based fillers are susceptible to hot cracks due to grease and dirt etc. ; the best cleaner in the world and suitable for welding is MEK (methyl-ethyl-ketone)

From weldability point of view, 309L or 309MoL is better choice since they have 3-8% of delta ferrite in the weld while inconel 600 does not.
During solidification this ferrite absorbs bad elements, such as sulphur, as a result it can prevent hot cracking.
For dissimilar joint, inconel is recommended because inconel shows similar thermal expansion of carbon steel.
If you have severe themal fatigue(heating and cooling frequently) condition, you may have fatigue cracking after a long use.
If you use your heat exchanger in stable condition(with light thermal fatigue) you may use 309L.
For tube to tube sheet(overlayed) welding, I do not expect thermal fatigue situation. Therefore, I recommend 309(Mo)L electrode. 309(Mo)L is a good high temperature material(Please consult Designer).
If you have to use inconel rod, please try ERNiCrMo-3 or ERNiCr-3 of other brands. Cracking susceptibility sometimes depends on brand.