Before you read on, I'm assuming that Rising Storm will be similar in that it will mainly focus on the early parts of the Pacific War, and I'm also only posting the following as "food for thought".
Now, wasn't one of the reasons for the U.S. Marines not being issued M1 rifles the fact that most were being shipped out as part of "Europe First" when the U.S. entered the war? And thus many had no choice but to use the Springfield, or that M1917 Enfield until more M1s became available.
It may be outside the timeline due to it being designed around 1944, however, as I understand it, the Japanese Type 4 rifle was meant to be based on the M1, but with modifications; the main modification being the use of two 5 round stripper clips rather than an en bloc clip, I imagine so they could share with other infantry and simplify production.
For balance purposes, I could imagine it being somewhat like this.
U.S. :
Rifleman: Springfield/M1917 Enfield (both are Bolts)
Assault: M1 Garand/M1 Carbine/"Chicago Typewriter"
Support: M1917 Browning Machine Gun/BAR
Commander: M1 Carbine/"Chicago Typewriter"
Sniper: M1903A4 Springfield
Japan:
Rifleman: Arisaka Type 99
Assault: Type II smg/Type 100 smg/Type 44 cavalry rifle
Support: Type 11/Type 96 lmg
Commander: Type 100 smg/Type 38 carbine
Sniper: Arisaka Type 99
A few side notes:
1.) Japanese should be able to achieve a higher rate of fire with any Arisaka rifle over any other nation's bolt-action due to its design.
2.) Japanese should be immediately issued bayonets for use.
3.) The Type 100 smg and Type 96 lmg were able to mount bayonets.
4.) As far as early war tanks goes, the Type 97 "Shinho To" Chi-Ha should prove to be on par (slightly inferior in armor) to the M1/2 Stuarts. Which was about all the Marines could field without getting heavily bogged down at some point.
5.) For fun, Japanese commanders could be given the option to wield a sword as a secondary rather than the Nambu.
Just my two cents. Food for thought~