Well, I got to say that Lom is indeed correct about his statement of filling in weak areas where Guard lacks... However, that is only just one of the reasons.
What you should think about, when including a small portion of WH force into IG, is what elements/tactics do you want to gain from employing those units?
Assassins are great as disrupting units in enemy army which can allow IG force to focus on other tasks or even to further enforce offensive tactics. (Good for Armour Assault tactic which is based on Hellhounds + LR Demo, Light Infantry army on offensive strategy with raining down close-to-long-range shooting, and also Drop Troops army going for offensive strategy)
Sister of Battles (SoB) can provide a high degree of anti-horde firepower within close-range gunfight. As you probably know, they are also resilent in which they can withstand most incoming fire from enemy weps. (As long as the weps are not AP2-3) Footslogging version are great as defensive units to protect IG army. Mechanised version, however, is now a mobile unit of tactical hvy troops. Sometimes, they can even be used as counter assault units to tie up enemy assault units that are about to attack IG weak units. (Thanks to their power armour) As a result, this allows IG units to focus on shooting their long-range weps.
Inquistorial Stormies with chimeras can be taken to add more mobile fire support to the IG army. (Possible to have quite a fair number of chimeras to conduct wall formation) With Rhinos, however, such units now become tactical mechanised troopers. It is possible to counter assault with them, but they are not as resilent as SoB. Although they can be used to assault enemy weak units in cover & tie them up thanks to their frag grenades.
Inquistor retinues are an interesting unit which you can equip them in so many ways. From shooty version to assault force. It is pricy, but this is a good opportunity for conversion & fluffiness. Not to mention that you can take Land Raiders to go along with the Leman Russ...
Having said all that... You would still have to judge whether it is a worthwhile or not for carrying out certain tactics with certain WH units. I can say that an inclusion of Inquistorial Force into IG is like an art which is opening up more possibilitie in tactics. Again, all of which have their own strength & weakness...
(In other words, there is no best way. You may have to find the right combination which suits your style of playing)
And about the smallest & effective force? I would probably think it to be a small number of SoB units...