If it would be practical for me to call in favours from people in the neighbourhood, and collect a multi-person team to perform bdikat hametz with me, would that be a worthwhile humra to take on myself? Specifically, I'm worried about the risk of rats moving leavened food to rooms that have already been checked, from rooms that have not, during the course of the bdikah process itself. With a team large enough that one person could be deployed to each room, the process could be conducted concurrently, in parallel, perhaps mitigating the risk. If it's relevant to the question, my apartment has a serious rat infestation problem.
The Mishna in Pesachim (1:2) states:
(Source(s) provided by Sefaria.org)
אֵין חוֹשְׁשִׁין שֶׁמָּא גָרְרָה חֻלְדָּה מִבַּיִת לְבַיִת וּמִמָּקוֹם לְמָקוֹם, דְּאִם כֵּן, מֵחָצֵר לְחָצֵר וּמֵעִיר לְעִיר, אֵין לַדָּבָר סוֹף:
We do not need to be concerned [that] perhaps a weasel dragged [chamets] from [one] house to [another] house, or from [one] place to [another] place. Since if [we were to be concerned, we would also need to be concerned that chamets was dragged] from [one] courtyard to [another] courtyard, and from [one] city to [another] city — there [would be] no end to the matter.
This being said it is to be noted that according to the Tiferes Yisroel (Yachin) notes the following:
(Partial quote, translation by myself)
ומעיר לעיר אין לדבר סוף דבשלומ' באותו בית מחדר לחדר יש לדבר סוף שהיה יכול לתקן שיבדקו כמה בנ"א כל החדרים ברגע א', אבל א"כ נחוש נמי שתביא מבית שבחצר זה לבית שבחצר אחר וכו' ואין לדבר סוף. ואע"ג דעכבר מצוי טפי, נקט חולדה לרבותא, אף דמצוייה שם חולדה דמעיזה טפי לבוא מיד אחר הבודק, אפ"ה לא חיישינן:
From [one] city to [another] city]- there [would be] no end to the matter: for it makes sense that in that home [when a weasel dragged chamtes] from room to room, there is an end to the matter, for it is possible to fix that many people search all the rooms at one time, but if so,we should also concerned that [the weasel] brought [chamets] from one house in that is in the courtyard to another house in the courtyard etc. [- Then] there would be no end to the matter. and even though a rat is more common [than a weasel, the Mishna] opts [for] weasel to include, even if weasels are common there [in that area where the chamets is being searched] which are more aggressive to come [into the room] immediately after the one searching the room [for chamets], nevertheless we are not concerned [that the weasel brought chamets into the room]
Thus, based on the above, it does not appear that it would be a worthwhile chumra to collect a multi-person team to perform bedikas chamets with you. For if you had a weasel infestation you would not need to be concerned about finding chamtes brought in by them. But, always be sure consult your local Orthodox Rabbi too to be certain .
Hope this is helpful!!