What's wrong with 6-7 ft tonearm wires?

Nil -- What was the capacitance per foot of the cables?

Best regards,
-- Al

Would the capacitance be too great in phono cables this long?

It depends on the cartridge type, the parameters of the particular cartridge, the capacitance per unit length of the particular cable, and ability of the phono stage to gracefully handle ultrasonic resonant peaks.

If the cartridge is a low output moving coil, the response of the cartridge itself within the audible spectrum will be pretty much insensitive to load capacitance. However, greater load capacitance will increase the amplitude and lower the frequency of an ultrasonic resonant peak, which may result in phono stage distortion products that fall within the audible spectrum. See the excellent post by Jonathan Carr (JCarr) dated 8/14/10 in this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1281468389

If it is a moving magnet cartridge, load capacitance that deviates significantly from the manufacturer's recommendation will cause frequency response in the upper treble to be non-flat. It is possible to have too little capacitance as well as too much.

Assuming you are referring to an lomc, if you choose a very low capacitance cable (e.g., 10 to 20 pf per foot), that also has quality shielding (to minimize noise pickup), fwiw my suspicion is that you'll be ok.

Regards,
-- Al

Could you please list some phono cables with those very low capacitance values?

Several of the Cardas phono cables shown on this page. Click on the corresponding photos:
Neutral Reference Phono: 20 pf/ft
Golden Presence Phono: 12 pf/ft (rca); 7 pf/ft (xlr)
Golden Reference Phono: 11 pf/ft

Several of the Kimber phono cables linked to on this page:
TAK CU: 47.1 pf for a 1 meter cable, apparently even including connector capacitance
TAK H: 46.9 pf for a 1 meter cable, apparently even including connector capacitance
TAK Ag: 47.1 pf for a 1 meter cable, apparently even including connector capacitance

My understanding is that the cartridge signals are 'balanced' by nature and RCA or XLR do not change this?

Connecting the cartridge to an rca phono stage input will convert it to an unbalanced source, because the impedances between each of the two cartridge output polarities and circuit ground in the phono stage will no longer be the same. That will negate the common mode noise rejection advantages that a balanced input stage would provide, but on the other hand it may not matter.

Good luck!
-- Al