Also, I have found that the rumor about all-same-weapon-squads learning their weapon skills faster is likely to be true.
Hmm, i found that uniformity vs. mixed-weapons is unlikely to matter for training efficiency:
mixed-weapons melee squad that has been training/sparring for a few years, six veterans, four rookies:
Legendary Swordsdwarf, no other weapon skills
Legendary Axedwarf, Competent Speardwarf (can't remember whether the dwarf immigrated with spear skill or switched weapons during training)
Legendary Swordsdwarf, no other weapon skills
Legendary Macedwarf, Novice Marksdwarf (Rusty; immigrated with marksdwarf skill, evidently never trained it)
Legendary Hammerdwarf, no other weapon skills
Legendary Speardwarf, no other weapon skills
Novice Axedwarf, Dabbling Speardwarf (picked up a spear first, equipment is "individual choice, melee")
Adept Swordsdwarf, Adequate Speardwarf (see above)
Novice Axedwarf, no other weapon skills
Great Speardwarf, dabbling Marksdwarf (immigrated with Marksdwarf skill, never trained/used)
There's a very tiny bit of cross-training applied to the three dwarfs who picked up a bit of spear experience in addition to their final weapon. Since the veterans handily reached high legendary skills in three years or so (legendary +20 or higher by the looks of the sparring logs), that cross-training doesn't hinder training in the main weapon and potentially improves mental attributes by training student, teacher, concentration and organisation skills. Not to mention that mixed-weapon squads are much more versatile when used in combat.
Cross-training was and is rare and takes up very little training time when it happens; it never was much of a concern to begin with and should be a complete non-issue with the faster training of DF 2014.
In short: build your squads however you please and don't worry about "will my dwarfs waste time telling the spear-users how to swing an axe?" a) usually they won't. b) even if they do, it hardly matters. What holds back sparring are typically lack of skills (dwarfs are reluctant to spar with less-than-adequate skills) and poor attributes (dwarfs that are quick to tire quickly abort sparring sessions). Once dwarfs get past those hindrances, they skill up very quickly, no matter how their squad is composed.