Hi TONY-S, just read your thread/post...
Yes, as you have found out, and others have pointed out, you will need an Interfit Flex Mount for each light modifier if you want to use each modifier simultaneously. The Flex Mounts take a bit of practice to get on/off the speed light smoothly; but you may find if you gently stretch the Flex Mount's inner perimeter surface a bit (as it is a rigid but somewhat flexible black polypropylene(?) plastic) with your hands will help future mounting/dismounting from the speed light.
CAVEAT – there are different Interfit Flex Mounts depending on which speed light the Flex Mount is to be attached to.
i.e. for your two Canon 580EXII it’s INTERFIT’s Mfr# SGM700:
http://www.bhphotovideo.com …GM700_Flex_Mount_for.html
but for your Vivitar 283 maybe it's INTERFIT Mfr# SGM500?(which fits the Vivitar 285HV)
http://www.bhphotovideo.com …GM500_Flex_Mount_for.html
I have both the INTERFIT PORTRAIT KIT(Mfr# STR101) & EFX KIT(Mfr#STR101; for location/travel), purchased about 5 years ago and while it is a great strobist kit, I find the 80/20 rule applies… (well actually a twist on the 1/3 rule) 80% of the time I am just using 2/6’s of the kit - the nifty Grid and Snoot and rarely use the other stuff in the kit. The rigid snoot is a bit cumbersome for location or travel with, but has great 20&30 degree grids for the snoot’s end to prevent light spillage and potential camera lens flare. And only occasional use of the Barn Door, but that's just me.
The major impediment I found with the Interfit Kit’s Flex Mounts is it’s a bit cumbersome to mount color gels (such as the inexpensive Rosco gels strips which most of us have just taped on to the face of the Canon speed light over the years). Color gels work with the Flex Mounts it’s just somewhat cumbersome.
While I still find the Interfit items very useful, I find the newer “mouse trap” by Expo Imaging’s ROGUE product line-
http://www.bhphotovideo.com …s=p_OVER_ALL_RATE%7c1&N=0
Rogue Grid:
http://www.bhphotovideo.com …ROGUEGRID_Rogue_Grid.html
and
Rogue fabric Flash Bender-Large: (used as a snoot)
http://www.bhphotovideo.com …r_Large_Positionable.html
a more versatile and more location/transport & travel friendly light modifier accessories that can be thrown in camera or light stand bags and sets up/tears down faster.
Rogue’s product design ingenuity & organizational pouches are very convenient.
Also, the often innovative UK manufacturer Lastolite introduced in approximately fall 2011, a new STOBO KIT system (but haven’t tried it):
http://www.lastolite.com/strobo.php
(which appears to handle color gels relatively well, as well it’s product design appears to take the light modifiers load bearing weight off the speed light and transfers it to the light stand, unlike the Interfit or Rogue)
http://www.lastoliteschoolofphotography.com/using-the-strobo-system
(26:10 minute video)
Have fun TONY - it’s a nice product Interfit!
[Interfit & Rogue both make very useful strobist stuff and everyone has to find what works best for them]
This is just one viewpoint, but I hope it may help any potential strobist purchasers in their product research.
Mary