If anyone is going to use Slug Sage Mode in this battle, I believe it's going to be Tsunade, not Sakura. She's been with Katsuyu for far longer than Sakura has, which gives her a time frame to have trained for it. Sakura doesn't really have that.
We can try and argue that she squeezed in the training for it between arc A and arc B, but considering her lack of chakra reserves and knowing that she's been working at less than 100% in order to learn Byakugou, I can't see it. The pieces of the puzzle don't fit.
She literally just accomplished Byakugou... I can't really see her pulling a perfected Slug Sage Mode out of her pocket, but I can see Tsunade revealing that such a jutsu does exist. Tsunade may not have completely mastered SSM, the same way that Jiraiya wasn't able to perfect Toad Sage Mode, but that will leave room for Sakura to master it herself. 
If Tsuande was capable of using Slug Sage Mode then why would she not activate it whilst she and the other Kage were fighting for their lives against Madara? It would be a nonsensical deus ex machina with no prior foreshadowing offered. I have heard several plausible theories that suggest that due to Tsunade's age reversal transformation technique consuming a considerable amount of her strength and concentration to maintain that she was unable to acquire SM. That could account for Tsunade developing the Byakugou technique as a replacement. According to some suspicions it's theorized as being either an imperfect variant of SM or a replicate ability that is akin to the Curse Seal. Regardless, it seems improbable that Tsunade would have this unfounded transformation when she has already revealed what is likely the extent of her abilities.
Not only that, it defeats the concept of Sakura surpassing Tsunade if her teacher reveals a powerful new ability that she is inherently incapable of ever attempting. Oh, and by the way, the Byakugou seal is an accumulation of enormous reserves of chakra which the user has been acquiring for some time. The notion that Sakura has deficient reserves became resolved the moment that seal developed on her forehead. Not that the possibility of her being able to compensate for her average levels with her precise chakra control was ever confuted.
Why should the amount of interactions that a character has had with the summonings be a concern? Naruto had only ever interacted with Gamabunta and the frogs on two different occasions before he was recruited by Fukasaku to learn senjutsu. Jiraiya probably fought alongside them for decades now, having developed a close relationship with the two sages who endeared him with a nickname IIRC and he likely trained with them for years. But did that somehow invalidate the fact that Naruto was able to not only acquire SM, but take it even further beyond than Jiraiya did in only a fraction of the time with much less interaction with the frogs?
Finally, there remains the two most significant qualities that implies or at least presents the possibility of Sakura having developed SM. The naming of her seal is apparently quite different to that of Tsunade's and she has yet to release it. Now does this mean that Sakura's Byakugou will become SM when she activates it? It's doubtful considering that she has been focusing primarily on just gathering chakra, but it's a possibility, especially if you consider the Byakugou jutsu to be an imperfect SM. Are there any regulations that claim that Sakura is incapable of acquiring new abilities as she concentrates chakra into a single area? No, of course not.
On to the second quality. You may find it to be somewhat contrived in nature but you do have to admit that the marking on Hashirama's forehead when he was entering SM bears a striking resemblance to Sakura's clan symbol. As we all know, in most situations, these resemblances aren't just a coincidence rather they are allusions to concepts or ideas that were previously established in the manga. I refer to one of my earlier posts that denoted how coincidence cannot be so easily discarded through explaining how the Naruto's symbol was suspiciously woven onto Konoha's uniforms for hundreds of chapters before it was finally revealed that there is indeed a substantial reason for its placement relating to the Uzumaki Clan's close political and military ties to the Senju.
Now even though Sakura's clan symbol will more than likely not having nearly as much importance as the Uzumaki the similarities between it and Hashirama's forehead marking should not be ridiculed when considering how Kishi enjoys making references and capitalizing on the ideas he established years ago.
One other thing. If Sakura was forced to wait until a timeskip to acquire SM then what purpose would there be to her acquiring it if she never used it in the main story? It just becomes an ineffectual and meaningless gimmick that she acquired that will never be properly explored in an actual battle. Unless you believe that Sakura is capable of mastering it during an intermission between this arc and the final one. But then who would she properly demonstrate it against? Juugo? The final arc is inevitably going to be dedicated almost entirely to Naruto and Sasuke's personal conflict if Orochimaru doesn't survive the end of this. If Sakura was ever going to use SM then this current battle would be the appropriate time to use it.