The answer is, Yes. They can be mixed. The problem is integration will become an issue. This is because Microsoft products easily mesh with their own products and services. The same goes for Google products and services. For example, Microsoft one drive for business will integrate more smoothly with office 365 than google drive. There is no need to add complexities to a provided solution. Using functionalities that come with office 365, like SharePoint provides benefits like having better structural and organizational control.
More than that, you get better search functionality with metadata usage using the complete Microsoft package. Mixing and matching the office 365 plans may have some limitations at the plan level. Meaning, depending on the plan or package you pay for; you could get some hitches along the way. The office 365 for business, business essentials, and business premium each have a limit of up to 300 users while the enterprise plans are for an unlimited number of users. Therefore, if you are planning to switch it up, this could be something to consider.
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Some companies have resorted to purchasing licenses through volume license or office 365 to work with G suite. There could be some reluctance to move away from G suite or office 365 altogether because of familiarity and fear of losing data and files. As a solution to keep both, IT departments for various companies have found the purchasing of licenses to be a useful way to keep both cloud-based software on board. However, in the long run, it may get costly. This is due to the repeated buying of licenses.
MULTIPLE AT ONCE
Surprisingly, some companies go out of their way to increase the software burden. Having Google drive or G suite which functions very well with its own default storage system; the same goes for Microsoft. There are some who prefer to use Dropbox as well. A cloud-based storing system for corporates an individual that works similarly to drive and One drive. The idea is to use one service and stick to its services for optimum efficiency. However, not everyone feels as attached to a brand. The goal is to achieve maximum results with what they perceive as the best technology.
The problem with doing away with one (productivity model) and sticking to another is that, if the majority of users are against it them adoption becomes much harder. The majority will have problems using it, with navigation of the software and getting used to the software will take a longer time. Not because of the way the system works, or integration issues but mainly because of attitude. If the majority has expressed negative feelings towards something, more often than not maintaining the status quo makes for a smooth process, but not always for better results.
Small businesses with numbers under 30 have a harder time integrating the two systems. This is because of the overall voice of the participating users and integration, in general, is not favored. The reason why some may choose Dropbox over Google drive is that it is better at syncing with desktop files as well. Often, the desktop files are Microsoft office suite files. G Suite files work best online, and while they also efficient, it is difficult to overlook specific custom fonts and other nitty-gritty details only available on Microsft word or excel, as compared to Google sheets as well.
It is also important to note, business to business also have their own standard and expectations. It is easier to do business when the docs, and files being shared are on the same platform. It makes it easier to use and collaborate even on teams. When it comes to some of the new plans that Microsoft has incorporated, it is harder to ignore. For example, while Google calendar and Gmail are very popular; a school administration staff may prefer to work with Office 365 for education due to its particulars.